The present invention relates to a magnetic recording medium. In particular, the present invention relates to a magnetic recording medium having excellent thermal stability and durability in which a magnetic powder is well dispersed in a binder.
Magnetic recording media such as magnetic tapes and magnetic cards are prepared usually by applying a magnetic paint comprising a magnetic powder and a binder for the powder to a base such as a polyester film to form a magnetic layer. Recently, a finely divided magnetic powder having a large specific surface area is used in order to increase the coercivity, maximum amount of saturation magnetization, S/N and magnetic rocording density.
However, when a vinyl chloride copolymer resin such as vinyl chloride/vinyl acetate/vinyl alcohol, vinyl chloride/vinyl acetate/maleic acid or vinyl chloride/vinyl acetate/maleic acid/vinyl alcohol copolymer resin is used as the binder for the finely divided magnetic powder in the preparation of the magnetic paint, problems such as an increase in the viscosity of the paint and insufficient dispersion of the finely divided magnetic powder in the binder are posed.
Under these circumstances, a surfactant having a low molecular weight was used as a dispersant for improving the dispersibility of the finely divided magnetic powder. However, the amount of the dispersant is limited, since when it is used in a large amount, the durability of the magnetic recording medium is reduced and a magnetic head (hereinafter referred to merely as the "head") is stained. Thus, it has been difficult to sufficiently improve the dispersibility of the finely divided magnetic powder.
For improving the dispersibility of the magnetic powder, a process wherein a high shear force is applied thereto in the dispersion step is employed.
However, in this process, the viscosity of the paint is increased and the temperature is elevated to thermally decompose the vinyl chloride copolymer resin. Gaseous hydrogen chloride formed by the thermal decomposition deteriorates the magnetic powder to reduce the durability of the magnetic recording medium to thereby affect the reliability.
Further, it was proposed to introduce an amine-modified vinyl unit into the vinyl chloride/vinyl acetate/vinyl alcohol copolymer resin usually used as the binder so that the binder per se will acquire a dispersing power (see Japanese Patent Application Kokai Publication No. 61-172213).
However, this process has a defect that since the thermal stability of the vinyl chloride copolymer resin per se is seriously reduced by the modification with the amine, the reliabilities of the magnetic recording medium and the head are reduced.
A process for preventing the thermal decomposition of the vinyl chloride copolymer resins by adding a stabilizer for vinyl chloride resins, such as an organotin compound, e.g. dibutyltin laurate or dibutyltin maleate, to the resins has been known.
However, the organotin compounds act as catalysts for the crosslinking reaction of the isocyanate compound usually added to the binder and, therefore, the pot life of the magnetic paint is extremely reduced, the stability of the magnetic paint is deteriorated and the viscosity of the paint is extremely increased in the course of coating to reduce the dispersibility of the magnetic powder and the surface smoothness of the magnetic layer.
In addition, a process wherein the magnetic coating film is crosslinked with electron beams so as to prepare a magnetic recording medium having an excellent durability and free of the problem of a shortened pot life was proposed. However, no intended magnetic recording medium can be obtained because the binder and the additives not always exhibit sufficient performances.